ld warning message on macOS build

From https://oldkea.isc.org/ticket/5669.

When building on macOS, the message

ld: warning: directory not found for option '-L/usr/local/lib64'

appears a number of times.

The problem appears to lie in "configure.ac", where "-L$d/lib64" is added to the locations where the log4cplus library may be found. ("d" being the directory tree in which log4cplus was installed). It is suggested that the existence of this directory be checked before adding it to the list.